This invention relates generally to belt conveyor carrying idlers and more particularly to belt carrying or support idler units which are tiltable to permit the conveyor belt to move in a curved path.
Belt conveyors of the endless belt type have long been used to move a continuous stream of granular or particulate material, particularly where the feed rates are relatively constant so that the material will fill the belt at a relatively constant or uniform rate. In order to maintain material centered on the belt, the belts are generally formed to have a flat center section and upwardly sloping side sections and therefore the support idlers over which the belt rides and which are located at spaced intervals generally consist of three rollers in a conforming configuration. While the three idler rollers may be arranged with their axes in a plane, it has been found that better support can be obtained by having the center roller shifted laterally so that the rollers may be increased in length and provide better support.
When the conveyor extends in a straight line, the conveyor idler assemblies need not be concerned with lateral movement because the trough shaped center is automatically self centering when the belt is under tension as it is when transporting material. However, at times it is necessary to have the conveyor follow a curved path to change direction in a horizontal plane and when this occurs there is a differential tension in the belt between the inner and outer edges as a result of the curve. Thus if the conventional idler assembly were to be used on curves there would be a natural tendency of the belt to move up along the idler roller on the inside of the curve so that if the radius of curvature was other than very long with respect to the belt size, the belt would tend to run off the roller supports toward the inside of the curve. Thus it has been found necessary to provide some arrangement to provide a counteracting force on the belt to keep it centered on the rollers.
One approach to this has been to provide edge rollers such as those shown in U.S. Pat. No. 4,917,232 which engage the edges of the belt and retain it in position. However, such arrangements not only tend to produce excessive wear along the belt edges, but they are effective only when the radius of curvature is quite large so that the differential tension between the inside and outside edges of the belt is relatively small and the belt itself is relatively rigid to prevent any lateral buckling or folding as a result of the force on the belt from the edge rollers.
Another approach has been to tilt the belt by suspending the idler roller assembly from an upper pivot to allow the belt to swing back and forth under the load to provide a self-aligning support for the belt. This arrangement again only works in curves with a very large radius of curvature and requires a great deal of lateral space to allow movement of the suspended assembly for the necessary degree of tilt. Since such space is often not available, particularly in underground tunnels and other areas, it has been found desirable to have the belt tilt with a minimum of lateral movement. One such arrangement is disclosed in the present inventor""s U.S. Pat. No. 5,341,920, granted Aug. 30, 1994. With this arrangement, the idler assembly consists of a fixed support frame having upwardly extending wheel or roller supports carrying wheels which engage tracks in the side of each of the sloping side channels of the movable support frame to support the movable frame on each side of the center line of the belt. At one side which is on the interior of the curve, is mounted a side roller to limit belt movement, and this side roller rotates about an axis perpendicular to the adjacent side member. When the assembly is mounted on a curved portion of the conveyor, the belt, because of the differential tension between the inside and outside produced by the curvature, tends to ride upward to engage the side roller and this force, together with the shifting weight of the load of material on the belt, causes the frame to tilt until the forces are in equilibrium. Because movement requires some pressure engagement between the belt edge and the side roller, the result has been a tendency of the belt to wear and even begin to delaminate along the edge under long continued usage.
The present invention provides a self-adjusting carrying idler assembly of improved and simplified construction which minimizes the forces required to tilt the assembly when the belt is on a curve and which minimizes edge wear and other stresses on the belt itself.
In the preferred embodiment of the present invention a generally horizontal fixed frame is provided which is supported at the ends and includes a pair of frame members spaced apart and providing a support plate between them which is positioned near the center line of the conveyor. This support plate provides an adjustable mounting for a wheel bracket which extends upward and carries a pair of spaced support rollers or wheels having an axis of rotation perpendicular to the frame and parallel to the center line of the belt. Also mounted on this support bracket is a horizontal roller having a substantially vertical axis of rotation which serves as a guide and takes any thrust along the axis the conveyor.
A movable or cradle frame of hollow box members is mounted above the fixed frame and consists of a center section and two side sections each extending upwardly at an angle to the center section and welded together as a unitary piece. The center section has a slot in the bottom wall through which the mounting bracket extends so that the support wheels and guide wheel engage the inside surfaces of the box to allow relatively close tolerances as well as protection from foreign matter such as the material being carried on the belt to prevent possible damage or excessive wear for the wheels. Each section of the movable frame has a pair of laterally extending brackets which provide support at each end for the carrying idlers. One idler is carried on one side of the center section and the other two on the opposite sides of the two side sections, and in accordance with usual practice make an angle of about 40xc2x0 with the center section to define the shape of the trough of the upper or carrying portion of the belt. On the side section at the outer end of what is to be the inside of the curve is mounted a guide roller which is positioned to be engageable by the edge of the belt to provide the necessary force to tilt the assembly to the desired degree.
Since the movable frame is supported basically at only a single point, it is free to rock back and forth about that point, and to provide the necessary stabilization and angular positioning, a crank arm is connected between the outer edge of the support frame and the outer edge of the side section opposite the guide roller. This crank arm is of such a length that it would be close to vertical if the center section of the movable frame were exactly horizontal. However, since there is no need for the extra expense and complexity of a movable carrying idler arrangement when the belt is perfectly straight and horizontal, the support bracket carries an angle bracket which also serves as a flat stop plate and as a support for the guide roller. The stop plate is engageable by the upper wall of the box section of the center section when the unit is tilted about 5xc2x0 so that the movable frame is never tilted at less than the 5xc2x0 that would be the initial position at the entry or exit from a curve. On the other hand, the assembly can tilt up to a maximum of about 20xc2x0 in a typical application since any higher tilt would risk the loss of some of the material being carried off the outside of the curve.
With the present arrangement the forces required to tilt the movable frame and which are produced by the contact between the belt edge and the side roller are minimized so that the pressure on the belt edge tending the damage the belt is greatly reduced.
Another feature is that the position of the support bracket is adjustable longitudinally along the fixed frame so that the pivot point moves with respect to the center of the belt. This, in turn, varies the force required to tilt the movable frame and compensate for changes in the radius of the curve of the belt.
Further features and advantages of the invention become apparent upon the more complete description in the following detailed description and the drawings.